A conventional solid electrolytic capacitor consists generally of a porous metal electrode, of an oxide layer disposed on the metal surface, an electrically conductive solid introduced into the porous structure, an outer electrode (contact connection), for example a silver layer or a metal foil with a separator, and also further electrical contacts and an encapsulation.
Examples of solid electrolytic capacitors are tantalum, aluminium, niobium and niobium oxide capacitors comprising charge transfer complexes or manganese dioxide or polymer-solid electrolytes. The use of porous bodies has the advantage that, owing to the large surface area, a very high capacitance density, i.e. a high electrical capacitance in a small space, can be achieved.
Particularly suitable solid electrolytes are, owing to their high electrical conductivity, π-conjugated polymers. π-conjugated polymers are also referred to as conductive polymers or as synthetic metals. They are gaining increasing economic significance, since polymers have advantages over metals with regard to processibility, to weight and to the controlled adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important and industrially utilized polythiophene being poly-3,4-(ethylene-1,2-dioxy)thiophene, often also referred to as poly(3,4-ethylenedioxythiophene), since it, in its oxidized form, has a very high conductivity.
Practical development in electronics is increasingly requiring solid electrolytic capacitors with very low equivalent series resistances (ESR). The reasons for this are, for example, falling logic voltages, a higher integration density and rising clock frequencies in integrated circuits. Moreover, a low ESR also lowers the power consumption, which is advantageous particularly for mobile, battery-operated applications. There is therefore the desire to reduce the ESR of solid electrolytic capacitors as far as possible.
European Patent EP-B 340 512 describes the production of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of the cationic polymers thereof, prepared by oxidative polymerization, as a solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene), as a replacement for manganese dioxide or for charge transfer complexes in solid electrolytic capacitors, lowers the equivalent series resistance of the capacitor and improves the frequency behaviour owing to the higher electrical conductivity.
A disadvantage of this and similar processes is that the conductive polymer is obtained by polymerization in situ in the electrolytic capacitor. To this end, the monomer, for example 3,4-ethylene-1,2-dioxythiophene, and oxidizing agent have to be introduced into the porous metal body together or successively in the presence of solvents, and then polymerized. Such a chemical reaction is, however, undesired in the course of production of electronic components, since it is very difficult always to allow the chemical reaction to proceed identically in millions of small porous components, in order to produce capacitors of identical specification.
Another disadvantage of in situ polymerizations in the production of solid electrolytes for capacitors is that the oxidizing agents can damage the dielectric (oxide layer) on the metal electrode. The oxidizing agents used are generally transition metal salts, for example Fe(III) salts. The reaction products of the polymerization which remain in the electrode body after the polymerization are then not just the electrically conductive polymer but also the reduced metal salts, for example Fe(II) salts. It is possible to attempt to remove these salts by subsequent washing steps. However, this is complex and does not succeed completely, i.e. residues of the metal salts always remain in the electrode body. As is well known, transition metals in particular can damage the dielectric, such that the elevated leakage currents resulting therefrom significantly reduce the lifetime of the capacitors or even make it impossible to use the capacitors under harsh conditions, such as high temperatures and/or high air humidity.
Furthermore, the production process of solid electrolytic capacitors when an in situ polymerization is employed is very complex: a polymerization process (impregnation, polymerization, washing) generally last several hours, it is necessary under some circumstances to use potentially explosive or toxic solvents here, and very many polymerization processes are required in order to produce a solid electrolyte.
Monomers can also be polymerized electrochemically in the absence of oxidizing agents. However, the electrochemical polymerization requires that a conductive film is first deposited on the insulating oxide layer of the metal electrode. This then again requires an in situ polymerization with all the disadvantages detailed above. Finally, this layer then has to be provided with electrical contacts for each individual metal electrode. This contact connection is very costly and inconvenient in mass production and can damage the oxide layer. Furthermore, electrochemical deposition in the pores of the porous metal electrode is very difficult, since the deposition takes place primarily on the outside of the electrode body owing to the electrical potential profile.
In Japanese patent application JP2006287182, polymer solutions are adjusted to a pH of 5.4 to 8.1 in order not to corrode the dielectric of the electrolytic capacitor and thus to lower the ESR. The use of polymer solutions having a pH of 1.2 to 1.6 for producing solid electrolytic capacitors leads to very high ESR values.
In PCT application WO-A1-2007/031206, the solid electrolyte of an electrolytic capacitor is produced by means of a dispersion comprising particles of poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate with a mean diameter of 1-100 nm. For corrosion-sensitive dielectrics, such as aluminium oxide, dispersions having a pH of 6 are used in order not to damage the dielectric. Even though this process overcomes the above-described disadvantages of the in situ polymerization, there is a need to further reduce the ESR.
There was thus a need to provide a process with which the ESR of solid electrolytic capacitors can be lowered further without in situ polymerization.